Vibration damping devices designed for installation between components making up a vibration transmission system in order to provide vibration damping linkage and vibration damping support between the components are known in the art. One example is an engine mount for providing vibration damping support of an automotive power unit on the vehicle body.
In the field of automotive engine mounts, a high degree of vibration damping capability is required for improved ride comfort. To meet this need, there have been proposed a number of fluid-filled type vibration damping devices designed to utilize the flow action, such as resonance action, of a non-compressible fluid filling the interior. A fluid-filled type vibration damping device of this design has a first fluid chamber and a second fluid chamber that give rise to relative pressure fluctuations at times of vibration input and that communicate through an orifice passage, the first fluid chamber and the second fluid chamber being filled with a non-compressible fluid. Vibration damping action is produced through action such as resonance action of the non-compressible fluid flowing through the orifice passage.
One requirement of an automotive engine mount is to provide vibration damping of multiple types of vibration having different frequency and amplitude, depending on factors such as engine speed and vehicle driving conditions. However, while the orifice passage affords excellent vibration damping action of vibration having the frequency to which it has been pre-tuned, in the case of vibration of high frequency above the tuning frequency, extremely high dynamic spring may arise due to antiresonance, and the resultant appreciable drop in vibration damping capability can sometimes be a problem.
Accordingly, in the prior art there have been proposed structures that afford variable orifice passage length, such as that disclosed in Patent Citation 1 (JP-U 7-18046) for example. Specifically, the orifice passage is defined by two orifice-defining members assembled in relatively rotatable fashion and extends in the direction of relative rotation of the members to provide a construction whereby the length of the orifice passage is variable through relative rotation of the two orifice-defining members. According to this variable length construction of the orifice passage, it is possible to adjust the tuning frequency of the orifice passage and to provide vibration damping of several types of vibration of different frequencies.
However, a conventional variable orifice passage length structure like that disclosed in Patent Citation 1 requires that an electric motor for driving rotation of the orifice-defining members be provided to the outside of the mount assembly. This makes it necessary for the drive transmission member that transmits the driving force of the electric motor to the orifice-defining members situated inside the mount assembly to be passed through the fluid chambers, namely the first fluid and second fluid chambers, from the outside to the inside. Where the drive transmission member is arranged passing through the fluid chambers in this way, the sealing structure for the fluid chamber of necessity becomes more complicated, posing a risk of being less durable and dependable; and the mount manufacturing operation, e.g. the fluid sealing operation during attachment of the drive transmission member, is considerably more difficult, posing a number of unresolved issues in terms of commercial viability.
Patent Citations 2 and 3 (JP-A 5-1739 and JP-A 5-231469) disclose a structure in which the orifice passage is defined by a gap formed between the opposing faces of two orifice members so that the cross-sectional area of the orifice passage may be varied by moving these two orifice members closer together or further apart in the direction of opposition.
However, in structures for providing the orifice passage with variable cross-sectional area as disclosed in Patent Citations 2 and 3, the gap extends in the circumferential direction, so making the cross-sectional area of the orifice passage smaller necessitates making the gap correspondingly narrow, which creates the problem of excessively high fluid flow resistance due to the narrowness of the gap. Specifically, while it is theoretically possible to set the tuning frequency of the orifice passage to a lower level by constricting the gap, a problem encountered in actual practice is that flow resistance rises to the point that an adequate level of fluid flow is not readily assured, making it difficult to achieve the desired orifice action.
Additionally, varying the dimension of the gap necessitates relative displacement of the opposing faces that define the gap, in the direction of their opposition. Relative displacement of the opposing faces situated in opposition across a narrow gap is associated with inflow of fluid to between the opposing faces or outflow of fluid from between the opposing faces, and this inflow or outflow of fluid inevitably produces a high level of resistance. The resultant difficulty of varying the size of gap between the opposing faces in a rapid manner may pose problems in terms of ensuring adequate response speed and levels of drive force.